The present invention concerns a method for directional down-hole drilling. The invention also concerns a device for carrying out the method.
For directional drilling in rock, drift mining or blast hole and well drilling a drill support in the form of coil tubing is used, which means that instead of a series of conventional rigid drill tubes a flexible continuous drill tube with one proximal and one distal end is used. The flexible drill support is unreeled from a spool and fed successively into the borehole. To form a drill string, a continuous bottom hole assembly (BHA), which includes a drill machine, is fastened at the distal end of the drill support. A percussion unit in the form of a fluid-pressure activated hammer or similar reciprocal mass is arranged on the drill machine to apply a drill bit fastened to the drill machine axially against the opposing rock surface. The bottom hole assembly (BHA) usually also comprises some kind of positioning device, so-called measurement while drilling (MWD). A driving fluid intended for the motors included in the BHA is led via a passage extended through the drill support. The rock cuttings are transported away from the worked area and out of the borehole with the used driving fluid.
In order switch the contact points of the drill bit studs and to make them work on uncut rock, the drill machine must be indexed or gradually rotated around its axis between each blow of the drill bit. This is normally achieved by rotating or twisting the flexible drill support with some kind of rotating arrangement situated outside the borehole.
For directional control of the drill bit inside the borehole, there is a steerable motor arranged in the BHA that conveys motion through a universal joint or other suitably flexible part so that the drill bit can take on different angles relative to the drill support. The said BHA normally constitutes a fluid activated motor in the form of a so-called mudmotor that, like the other motors in the BHA, is supplied with a driving fluid via a passage extending through the drill support.
When drilling deep holes, so-called “stick slip” can arise due to the rotation of the drill support, which at its proximal end is continuous or uniform, becoming irregular at its distal end so that the drill support acts like a torsion spring instead of indexing the drill machine evenly between blows from the percussion unit. This means that the drill machine will be standing still for several blows and accumulating torque before running away uncontrollably while performing a very rapid rotation. This “stick-slip” effect reduces the penetration rate and increases bit wear.
One considerable difference between conventional linear drilling with drill support comprising a series of joinable drill tubes and a flexible drill tube of coil string type is that the use of a flexible drill support allows only a relatively small application force on the drill bit, considerably reducing the efficiency of the drilling technique. In particular with regard to directional drilling of curved holes, this limitation is tangible because a large part of the application force is lost in each part of the drill tube that bends. This problem is due partly to geometric conditions but also partly to the large static forces of friction that arise at each deflected part of the drill support. In this part, it should be understood that the aforesaid problems become greater as the curve radius becomes smaller.
Recently, it has become increasingly common to use directional drilling, which puts higher demands on equipment efficiency and especially when drilling series of non-linear holes, i.e. curved or crooked holes. One example of this is in cases where it is necessary to avoid drilling in non-ore bearing rock but to efficiently control the drilling direction directly towards the ore-bearing bodies in the rock.